This invention relates to a self-supporting aerial optical cable in which a non-metallic element is used, as compared to a metallic element, as a cable component.
As is well-known in the prior art, an optical fiber is formed of a dielectric material and therefore is not subjected to electromagnetic induction from a power line. As a result, a non-metallic optical cable, in which no metallic elements are employed as cable components, is frequently used in applications where it is necessary or practical for the optical cable to be mounted on the power line.
FIG. 1 shows a conventional non-metallic optical cable. An optical fiber core 11, as well as a cable component such as a plastic cord 12 or the like, is twisted around a tension member 13 formed of a fiber glass reinforced plastic rod (hereinafter referred to as F.R.P. rod) or Kevlar rod and the like. A shock absorbing member 14 is made of, for example, plastic yarn and is wound around the optical fiber core 11 to protect the latter against external pressure. A plastic tape 15 is then wound around the shock absorbing member 14 and is provided with a sheath 16 of polyethylene or the like circumferentially about the tape 15. The non-metallic optical cable according to such a configuration is never subjected to electromagnetic induction, as is an optical cable in which a metallic element, such as a copper wire or the like, is used as the tension member. The optical cable thus provides a measure of safety during maintenance and inspection, and it is not necessary to provide a ground for induced current, thereby simplifying the installation of the optical table.
When the non-metallic optical cable is used as an aerial cable, it tends to rely on a conventional hanger or suspension wire system. However, either a hanger or suspension wire is comprised of a metallic element, so that not only is the non-metallic optical cable compromised by the inclusion of a metallic element, but the cable may also be subjected to a positional restriction.
For these reasons, a self-supporting non-metallic aerial optical cable, as shown in FIGS. 2-4, has been designed which takes into consideration the increase in tension due to wind and snow to which the cable is exposed and also takes into consideration the high tension normally applied to a conventional self-supporting aerial cable.
In FIGS. 2-4, like numerals are used to designate like parts. FIG. 2 shows a non-metallic self-supporting aerial optical cable arranged so that a non-metallic optical cable core 20 and a suspension wire made of a non-metallic rod such as F.R.P. rod, with high Young's modulus, or cable and the like are arranged in parallel to one another, The optical cable and suspension wire are integrally covered with a plastic sheath 23 by disposing a neck portion 22 therebetween, and the suspension wire 21 supports the cable core 20,
FIG. 3 shows a modified form of a non-metallic self-supporting aerial optical cable arranged so that the non-metallic optical cable core 20 and the suspension wire 21 made of a nonmetallic rod are covered with separated plastic sheaths 24, 25, respectively. FIG. 3 also shows that an optical cable 26 is wound around the suspension wire 27 at a predetermined pitch.
Similarly to the modified form shown in FIG. 3, another modified form of the non-metallic self-supporting aerial optical cable, shown in FIG. 4, is arranged so that an optical cable 26 and a suspension wire 27 are covered with the separated plastic sheaths 24, 25 and are disposed in parallel to each other. In this arrangement, the optical cable and suspension wire are circumferentially wound by one or more Kevlar rods coated with plastic or a binding wire 28 of a high tensile fiber element, in order to have the suspension wire 27 support the optical cable 26.
The non-metallic aerial optical cables arranged as described above, create problems which will be discussed hereinafter.
In general, the F.R.P. rod or Kevlar rod has a Young's modulus of about5000 kg/mm.sup.2 -7000 kg/mm.sup.2, which is smaller than the Young's modulus of a steel wire, normally employed in a suspension wire, of about 20,000 kg/mm.sup.2. Thus the Young's modulus of an F.R.P. rod or Kevlar rod is about 1/4-166 of that of a steel wire. It is well-known that, in an optical cable, if excess tension is applied thereto for hours, static fatigue will result which will eventually lead to fractures. For this reason, the optical cable is required to have an elongation of less than 0.2% (guaranteed for more than ten year's life) of allowable tension on the cable. As a result, if the tensile force of an optical cable which is accompanied by a suspension wire such as an F.R.P. rod or a Kevlar rod and the like, is to be set for 0.2% elongation of the cable, and if the same condition (tension and slack) is provided in an optical cable with the conventional steel suspension wire, then a nonmetallic rod having a diameter of 1.7-2 times more than that of the steel wire will be required. This will, of course, present problems such as increasing the wind pressure and load, reducing the flexibility of the cable, and increasing the expense of the cable.